The receiving frequency band of a communication satellite receiver is 950 to 1770 MHz and a tunable filter for selecting a channel frequency has a circuit configuration shown in FIG. 14 which is disclosed in Japanese Patent Laid-Open 3-135211.
In FIG. 14, the tunable filter is composed of coils 4, 5, 19, 20, 21 and 22; capacitors 8 and 9; resistors 12 and 13; variable capacitance diodes 10 and 11 and the channel frequency is selected by applying a control voltage to the cathodes of variable capacitance diodes 10 and 11 through resistors 12 and 13, respectively. The frequency characteristics of the tunable filter are shown in FIG. 15, coils 19 and 21 resonsate with each of their stray capacitances and the image frequency is attenuated.
However, according to digitalization of broadcasting by broadcast satellite and communication satellite, the receiving frequency band is expanding, for example 950 to 1890 MHz in Japan and 950 to 2150 MHz in Europe.
In a usual tunable filter, it is impossible to vary the tuning frequency up to such an expanded high band frequency but if a tunable filter tunable over a wide band is composed anyway, it is difficult to work coils 19 and 21 as a trap and it becomes difficult to attenuate image frequency band. Moreover, when a channel frequency in a high band is selected, the foot of the frequency characteristic of a tunable filter becomes wider compared with when a channel frequency in a low band is selected and the exclusion ability of undesired signal decreases and the attenuation at the local oscillation frequency and the image frequency become worse compared with that at a low band channel reception. A frequency characteristic of a tunable filter in this state is shown in FIG. 13.
Therefore, it is desirable to provide a low pass filter, a trap circuit or the like, in which resonance frequency varies according to the tuning frequency, ahead of and behind the tuning filter and to attenuate the local frequency and/or the image frequency. This increases the cost of the receiver.